1. Field of the Invention
The present invention refers to a cooled mirror for laser applications and in particular to a cooled mirror with a mirror body having a mirror surface and comprising several layers with their surfaces connected in a stack-like arrangement and forming a cooler structure with connections for the inlet and outlet of a coolant, whereby a first layer forming the top of the mirror contains or forms the mirror surface. The present invention refers also to a method for the manufacture of the said cooled mirror for laser applications.
2. Description of the Prior Art
In laser devices, for example in such devices used for processing parts, mirrors are generally needed for reflecting the laser beam, and are moved by a drive for processing of a part. Although the laser beam is generally sent as a widened beam to the mirror used in order to reduce the power density, it is common practice to actively cool such mirrors, using a coolant that flows through a cooler structure of the mirror body, preferably with a liquid coolant, for example such as water. The widened laser beam is then focused in a focal point by means of a lens for processing of the respective part. The focal point must be of high quality, i.e. in particular a pre-defined diameter, for example a circular diameter, and there must be an extremely homogeneous distribution of the laser power within the focal point. The part can be processed to meet quality requirements only if these conditions are fulfilled by a respective high-quality beam.
Therefore, mirrors for laser devices or the mirror surfaces of such mirrors must possess a high degree of mechanical and thermal stability, i.e. high stability against external forces and/or pressure, against temperature deviations or deformations etc., thus avoiding any resulting alterations and/or deformations of the mirror surface. Miniscule alterations and/or deformations, such as warping of the mirror surface in the sub-micrometer range already cause a serious decline in the beam quality of the laser beam reflected on the mirror, especially a decline in the shape or diameter of the beam and the quality of the laser beam focal point or focus on a surface of the part to be processed. Furthermore, minimal deformations of a mirror surface cause significant operating time differences in the laser beam, thus also adversely affecting the quality of the laser beam focal point and therefore the quality of the processing of the part with the laser.
Numerous suggestions for cooled mirrors for laser devices are known in the art. All of these suggestions are unsatisfactory.
A cooled mirror for a higher power laser is known, as is described in U.S. Pat. No. 4,443,059. In this mirror, the mirror surface consists of a layer made from a material with high thermal conductivity, of which, however, the thermal expansion coefficient is adapted as nearly as possible to the material of the components connected to the cooled mirror and therefore in particular to the thermal expansion coefficient of the connected cooler structure. Therefore, the materials recommended for the layer comprising the mirror surface are molybdenum, tungsten, silicon carbide or silicon nitride. The cooler structure is made from a further plate that is provided with a plurality of grooves on one side that are divided by stays and form cooling channels for the coolant. By means of the stays, this cooler structure is connected directly with the plate possessing the mirror surface. The side of the plate comprising the cooler structure facing away from the mirror surface is connected with a support body with a honeycomb profile containing channels for the inlet and outlet of the coolant.
The disadvantage is the complex and also non-symmetrical design of the mirror, so that the axis direction is perpendicular to the mirror surface, causing thermal deformations (bimetal effect). Another disadvantage is that the individual stays of the plate forming the cooler structure connect directly to the layer possessing the mirror surface without an additional intermediate layer, which causes uneven cooling and therefore deformations of the mirror surface. A final disadvantage of the known mirror is that it has a high material mass, which is undesirable for laser mirrors that are moved for processing of parts.
The groove-like cooling channels used also have the disadvantage that the stiffness of the mirror on the mirror surface in the longitudinal direction of the grooves is greater than in the transverse direction. In the necessary finishing of the mirror surface, for example by means of milling with a diamond tool, this causes structures to be formed in the mirror surface due to the varying stiffness, which also adversely affects the beam quality. This is a disadvantage of all known mirrors that use cooling channels that extend parallel to the mirror surface.
A further light-weight mirror for laser applications is described in U.S. Pat. No. 5,002,378. For the cooling of this known mirror, however, there is no cooler structure with connections for the inlet and outlet of a coolant, i.e. the mirror is not connected to an external coolant circuit. In this case, the cooler design is based on the heat-pipe cooling principle.
A further mirror structure cooled by a coolant, is shown in U.S. Pat. No. 3,781,094, in which the cooler structure consists of a plurality of perpendicularly crossing cooling channels that are all limited by stays. The disadvantage of this known construction is again the lack of symmetry in the direction perpendicular to the mirror axis, especially also of the cooler structure, in addition to the manufacturing expense. Moreover, the stays limiting the cooling channels of this known mirror likewise extend directly to the layer possessing the mirror surface, so that no homogeneous cooling of the mirror surface is achieved, which means that thermal deformations and faults in the mirror surface are unavoidable.
In a further known mirror for laser applications, a plurality of separate cooling channels are formed by using corrugated plates in the cooler structure as shown in U.S. Pat. No. 4,387,962. This known construction is likewise complex and does not possess the required symmetry.
A further known cooled mirror for laser applications described in DE 33 39 076 A1 has an extremely complex cooler structure and consists essentially of a plurality of posts that extend from the bottom of a plate forming the mirror surface and that are subjected to circulation by a coolant emitted from nozzles. The disadvantage here is likewise the very complex and therefore expensive construction in addition to the lack of a symmetrical design.
A further known cooled mirror for laser applications shown in U.S. Pat. No. 4,770,521 has a cooler structure formed directly beneath a layer comprising the mirror surface. Here the layer comprising the mirror surface is separated by a further layer parallel to this layer by a plurality of polygonal spacers, so that a space is formed between the two layers for circulation of a coolant. The disadvantage of this known cooler is likewise a complex and expensive design. Since the spacers are directly connected with the layer forming the mirror surface, no homogeneous cooling of the mirror surface is achieved.
Finally, a known cooled mirror for laser applications described in U.S. Pat. No. 4,403,828 consists of a very massive block that is provided on top with several grooves, each of which forms a cooling channel. A plate bearing the mirror surface is attached on the top by soldering to the stays of the grooves. The disadvantages here include the high weight of the mirror, making it unsuitable for a large number of applications. Moreover, there is no symmetrical design.